Sorry to hear you didn't go with Grails (my framework of choice too). Sounds like the .Net choice was based on his skills, and not yours. The domain-first approach of Grails, especially with greenfield projects, is so tempting that it'd be really hard for me not to choose it for something these days. I have chosen non-Grails tech in the last year, but it's been a tough choice in those cases. :)
How's the project site coming along?